Various embodiments of the present disclosure relate to an automatic transmission for a vehicle, and more particularly, to a method of protecting a frictional element of a clutch for the automatic transmission, which can prevent the damage of the frictional element by calculating a rotation number of the frictional element and reducing the rotation number of an engine when the rotation number of the frictional element exceeds the burn time of the frictional element, in an automatic transmission which does not have a speed sensor for measuring the rotation number of the frictional element.